Magnetrons for dielectric heating



Dec. 21, 1965 (jJELID 3,225,302

MAGNETRONS FOR DIELECTRIC HEATING Filed March 6. 1962 Amplifier did 1U M 5, MM, mgww United States Patent Ofitice 3,225,302 Patented Dec. 21, 1965 3,225,302 MAGNETRONS FOR DIELECTRIC HEATING Gtiran Axel Runo Ojelid, Huskvarna, Sweden, assignor to Husqvarna Vapenfabriks Aktielbolag, Huslivarna, Sweden Filed Mar. 6, 1962, Ser. No. 177,866 Claims priority, application Sweden, Mar. 13, 1961, 2,631/ 61 16 Claims. (Cl. 328-270) This application generally relates to such ovens as form the subject matter of my co-pending US. patent application No. 107,994, filed on May 5, 1961, and relating to Timed switch for electrical baking and roasting ovens which is assigned to the assignee of the present application.

In such magnetrons, as are utilized in equipment for warming or heating by means of high frequency irradiation, as a rule 2-3 minutes are required for the heating of the cathode to its operating temperature. Particularly in respect of equipment for the preparation of food in eating-houses this is an inconveniently long time of Waiting, which easily leads to the serious condition, that the magnetron is maintained interconnected with its cathode at full operating temperature, in spite of the fact, that the magnetron only has to operate periodically, which entails undue Wearing of the cathode.

The prime object of this invention is to eliminate this drawback and to provide means for attaining a short time of heating of the cathode of magnetrons for dielectric heating.

With this and other objects in view a filament, which is in ordinary manner arranged in heat conducting relation to the cathode, is permanently connected to a current source, such as a transformer, the voltage of which is adapted to cause in the filament a heat generation which maintains the cathode at a readiness temperature well below the operating temperature but above the ambient temperature, in respect of barium cathodes preferable about 500 C., and the current source is through a switch, which is automatically operative upon the manipulation of a switch or another equivalent a manual measure or operation for the starting of the mag netron, adapted to supply a voltage substantially higher than the normal voltage, at least during a time sufiicient for the raising of the cathode to operating temperature, said switch also being adapted to prevent the application of anode voltage to the magnetron during the said time, since such voltage would injure the cothode.

Further objects and advantages of the invention will become more fully apparent from the following description and the accompanying drawing illustrating two preferred embodiments of the invention, and in which FIG. 1 is a circuit diagram illustrating preferred circuitry for rapid start of a high frequency magnetron.

FIG 2 represents a modification of the circuitry according to FIG. 1.

FIG. 1 discloses how a three phase alternating current supply system with a neutral conductor is adapted to be connected to a voltage increasing and rectifying assembly 1 and a filament current transformer 2 for driving a high frequency magnetron 3. The assembly 1 is connected to the supply system by means of a manual switch 5 and a contactor 6, the connecting as well as the disconnecting-of which is delayed by electronic equipment comprising a rectifier 10, two capacitors 11 and 12, a

relay 13, a thyratron 14 and resistors 15, 16, 17, 18, 19 and 20. Transformer 2 is in the OFF position of switch 5, connected to the neutral conductor and one phase line through a break contact of switch 5. Transformer 2 therefore supplies a current suitable for maintaining the cathode of the magnetron at a readiness temperature.

When switch 5 is set in its ON position, current is applied to rectifier 10 and relay 13, immediately after that the direct connection of transformer 2 with the phase line has become interrupted at the break contact of switch 5. This means that transformer 2, now interconnected through the break contact of relay 13 supplies a greater current to the cathode of the magnetron and on account hereof raises its temperature to the operating temperature simultaneously as the direct voltage provided by the rectifier 10 and the smoothing capacitor 11 slowly charges capacitor 12 through resistors 15, 16 and 17. During this time almost no current flows through the energizing coils of relay 13 and the contactor 6 respectively, since they are connected to balanced points of the network of resistors. When capacitor 12 has reached a certain voltage, however, thyratron 14 fires, which causes unbalance in the network. Since relay 13 has less mass than contactor 6, the relay will be actuated first by the unbalance. This means firstly that resistor 20 will become interconnected in the network, and secondly that transformer 2 will again become interconnected to supply a reduced current to the filament. The interconnecting of resistor 20 makes the network become in balance with respect to the connecting points of the energizing coil of contactor 6, when thyratron 14 is conducting. Contactor 6 therefore remains unactuated until thyratron 14 is extinguished, which is most conveniently brought about through momentary breaking of its anode circuit by means of a manually actuatable switch 40. When the thyratron is extinguished, relay 13 remains energized, and the unbalance occasioned gives rise to a current through the energizing coil of contactor 6, which closes the make contacts of the contactor and thereby connects assembly 1 to the main supply system and initiates the operation of magnetron 3. When thyratron 14 fires again on account of the charging of capacitor 12, contactor 6 is reset and interrupts the operation of the magnetron. Through adjustment, possibly by means of push-buttons, of the resistance of resistor 17, a suitable period of operation may be set in advance.

The described delay of the operation of contactor 6 during a starting period, when a greater current is applied for the heating of the cathode of the magnetron, involves an activation of the cathode which after that remains completely operative during several hours in spite of the fact that it becomes heated to full operation temperature only during the working periods of the magnetron, which heating then is accomplished by the effect of the anode current upon the cathode.

FIG. 2 represents a modified rapid start circuit, particularly adapted for greater or high power, high frequency magnetrons and operating with greater accuracy. The circuit according to FIG. 2 comprises, in addition to assembly 1, transformer 2 and magnetron 3 a manually actuatable switch 7 without a break contact, a contactor 8 for alternating current and having three make contacts for the phase lines and an auxiliary break contact, and a choke coil 9. The electronic equipment for delaying the operation of the contactor comprises, in addition to rectifier 10, capacitors 11, 12, resistor 17 and thyratron 14,

also a voltage stabilizing glow lamp 21, two relays 22, 23, each having a make contact and a break contact, two diodes 24, 25, four resistors 26, 27, 28, 29, two push button switches 30, 33, of which the latter is an automatically returning HF-button, and a signal lamp 32.

When switch 7 is not actuated, transformer 2 receives a limited portion of the phase voltage of the main supply system through choke coil 9 and the break contact of contactor 8. Choke coil 9 is so proportioned, that this portion maintains the cathode of magnetron 3 at a readiness temperature. The voltage is also sufiicient for maintaining a discharge in the glow lamp 21 and for charging capacitor 12 through resistors 17 and 25 to a voltage determined by the potentiometer resistor 28. The said voltage is a little lower than the breakdown voltage of thyratron 14. The voltage is, however, not suflicient to operate relay 22.

When switch 7 is manipulated, choke coil 9 becomes bridged, and by means of a mechanical interconnection also switch 30 is thereby opened. This means that transformer 2 has full voltage applied to it and rapidly raises the temperature of the cathode of the magnetron to operating temperature, and that the voltage over capacitor 12 simultaneously is permitted, through the disconnection of di-ode'25, to rise to the break-down voltage of thyratron 14. By adjustment of the potentiometer resistor 28 with respect particularly to resistor 26, the time of raising of the voltage of capacitor 12 may be pre-set in agreement with the time required for raising the temperature of the cathode of the magnetron, that is about seconds. When thyratron 14 fires, the two relays 22 and 23 are energized with the consequence that the energizing coil of contactor 8 remains free of current and that current starts flowing through diode 24 to t-hyratron 14 and capacitor 12. Lamp 32 is also lighted to indicate that the oven is ready for operation. If the thyratron should now be extinguished through momentary opening of its anode circuit by means of pressing the push button switch 33 to cause desired magnetron heating, all current through the thyratron ceases and relay 23 is released with the consequence that contactor 8 has energizing current applied to it and closes its make contacts and opens its break contact. Relay 22 remains namely energized on account of the current through tube 21. The magnetron then starts operating, and it does this without any filament current, since the magnetron is of such a magnitude, that its cathode is suificiently heated by the effect of the anode current. To enable the circuit to be utilized also in smaller magnetrons, the break contact of contactor 8 may be bridged by a suitable resistor. When the thyratron became extinguished, the voltage of capactor 12 began to raise of account of the. current flowing through resistors 27, 2 8, diode 24 and resistor 17. In this connection resistor 26 plays an extremely unimportant role, since it has a very great resistance in relation to resistors 27 and 28. The rate of the increase in voltage depends, besides upon the setting of resistors 28 and 29, upon resistor 17, which in the first instance is adjustable for the selection of the duration of the time of operation. When the t-hyratron fires again, relay 23 becomes energized and breaks the operating current for contactor 8, which is hereby reset.

If an interruption of the current supply from the main supply system should occur, the magnetron is fully protected against the supply of anode current until its cathode have had time to reach its readiness temperature, which requires about 75 seconds, and in addition thereto to reach its operating temperature, since capacitor 12 in such an instant is quickly discharged through resistor 17, diode 25 and resistors 28, 29. When voltage returns, capacitor 12 can only receive current through the very high resistance 26, and its voltage will therefore rise so slowly, that the cathode of the magnetron has time to become heated before the firing of thyraton 14. Glow lamp 21 stabilizes the readiness voltage of capacitor 12 as well as the voltage used for charging capacitor 12 during the intervals of operation, which is important for the obtaining of correct starting and working periods at normal variations in the voltage of the main supply system.

Tabulated below are the values of the most important components of the circuitry according to FIG. 2.

Capacitor 11 ,u.f 8 Capacitor 12 /.Lf 6 Resistor 17 oh1n 15OM Resistor 26 do 40M Resistor 27 do 68K Resistor 28 do 500K Resistor 29 do 50K These values are, however, given only to clarify the invention and should not in any way be construed as limitations.

While the invention has been particularly shown and described with reference to some preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What I claim is:

1. In apparatus for dielectric heating:

a magnetron having an anode, a cathode and a filament for heating said cathode,

a current source for said filament,

a voltage source for said anode,

means supplying to said filament from said current source a current sufficient to maintain said cathode at an elevated readiness temperature above the ambient temperature but below its ordinary operation temperature,

means temporarily increasing the current through said filament during the starting period of the magnetron to thereby raise the temperature of the cathode to a heating temperature, which is at least equal to its ordinary operating temperature during said starting period, said means increasing the current including manually actuatable switch means connected to said voltage source, and

means for preventing the application of anode voltage to said anode before the expiration of said time interval.

2. An apparatus as claimed in claim 1, comprising a lamp which is connected to said manually actuatable switch means when said filament has attained operating temperature, said lamp indicating that said filament has attained its operating temperature and that the magnetron is consequently ready for operation, and a second manually actuatable switch which has an automatic return movement for connecting the magnetron to said voltage source when said lamp has become lighted, said second switch being then connected to said manually actuatable switch means.

3. Apparatus as in claim 1 comprising means automatically reducing the current through said filament not later than the application of anode voltage to said anode.

4. Apparatus as in claim 1 comprising means automatically shutting off the current through said filament not later than the application of anode voltage to said anode.

5. Apparatus as in claim 1 comprising manually settable means selecting in advance a predetermined time of opera-- tion of the apparatus.

6. Apparatus as in claim 1 wherein said filament current increasing means includes timing means for determining said interval, said apparatus further comprising means including said timing means for determining the length of time said anode voltage is applied to the anode.

7. Apparatus as in claim 1 including balanced bridge means,

means for unbalancing said bridge means at the end of said time interval,

means responsive to unbalance of said bridge means for decreasing the current to said filament and for compensating for unbalance of the bridge means,

means coupled to said bridge means for connecting said anode to said voltage source, and

means for interrupting said unbalancing means to cause said connecting means to operate and apply voltage to said anode for said time interval.

8. Apparatus as in claim 7 wherein said unbalancing means includes a thyratron.

9. Apparatus as in claim 7 wherein said current decreasing means causes the cathode temperature to be lowered to a value between said readiness and heating temperatures.

10. Apparatus as in claim 1 wherein said preventing means includes actuating means for connecting said voltage source and anode and second and third switch means serially connected together between said voltage source and actuating means,

said second switch means having a normally closed contact and said third switch means having a normally open switch contact,

said filament current increasing means including timing means operable in response to the closing of said actuatable switch means for reversing said contacts respectively to open and closed positions at the end of said time interval, and

means operable after said interval to close said normally closed contact for causing said actuating means to connect said voltage source and anode.

11. Apparatus as in claim 10 and including means comprising said timing means for determining the length of time said anode voltage is applied to the anode.

12. Apparatus as in claim 10 including a normally open energizing line having respective means serially connected for operating said second and third switch means when energized by said line,

said timing means being coupled to close said line at the end of said interval,

said means operable thereafter to close said normally closed contact being effective to accomplish same by re-opening said line to deenergize said second and third switch operating means, and

means for preventing deenergization of said third switch operating means by said line re-opening, thereby maintaining said third switch means closed. 13. Apparatus as in claim 1 wherein said preventing means includes first relay means having respective switch 5 means, and

second relay means having respective switch means coupled to said first relay switch means,

said first and second relay switch means having respective contacts one of which is normally open and the other normally closed,

timing means for energizing said first and second relay means at the end of said time interval respectively to close and open said contacts, and

means for deenergizing at will only the one of said relay means whose contact is normally closed to cause said relay means to effect application of anode voltage to said anode.

14. Apparatus as in claim 13 wherein said deenergizing means includes means for preventing deenergization of the relay means whose contact is normally open.

15. Apparatus as in claim 13 wherein said relay means have respective coils, and including an energizing line having a thyratron serially connected to the coils of said relay means,

said timing means being operative in response to the closing of said actuatable switch means to fire said thyratron at the end of said interval and thereby energize both said relay means,

said deenergizing means including a switch in said line to open same and deenergize said thyratron and relay means, and

means for preventing deenergization by said switch of the relay switch contact that is normally open. 16. Apparatus as in claim 15 wherein said timing means 35 1ncludes a condenser and charging and discharging errcuits.

References Cited by the Examiner UNITED STATES PATENTS 2,104,844 1/1938 Aifel 315-102 GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, Examiner. 

1. IN APPARATUS FOR DIELECTRIC HEATING: A MAGNETRON HAVING AN ANODE, A CATHODE AND A FILAMENT FOR HEATING SAID CATHODE, A CURRENT SOURCE FOR SAID FILAMENT, A VOLTAGE SOURCE FOR SAID ANODE, MEANS SUPPLYING TO SAID FILAMENT FROM SAID CURRENT SOURCE A CURRENT SUFFICIENT TO MAINTAIN SAID CATHODE AT AN ELEVATED READINESS TEMPERATURE ABOVE THE AMBIENT TEMPERATURE BUT BELOW ITS ORDINARY OPERATION TEMPERATURE, MEANS TEMPORARILY INCREASING THE CURRENT THROUGH SAID FILAMENT DURING THE STARTING PERIOD OF THE MAGNETRON TO THEREBY RAISE THE TEMPERATURE OF THE CATHODE TO A HEATING TEMPERATURE, WHICH IS AT LEAST EQUAL TO ITS ORDINARY OPERATING TEMPERATURE DURING SAID STARTING PERIOD, SAID MEANS INCREASING THE CURRENT INCLUDING MANUALLY ACTUATABLE SWITCH MEANS CONNECTED TO SAID VOLTAGE SOURCE, AND MEANS FOR PREVENTING THE APPLICATION OF ANODE VOLTAGE TO SAID ANODE BEFORE THE EXPIRATION OF SAID TIME INTERVAL. 